1. The Field of the Invention
The present invention relates to image reproduction systems and methods, and in particular, to a system and method for color image reproduction that utilizes a multi-level process with level limiting capabilities using a digital computing system.
2. The Relevant Technology
The colors seen in reproductions contained in most magazines, newspapers, colored brochures, and the like are typically created from an optical illusion which results from a process called xe2x80x9chalf-toning.xe2x80x9d In these publications, a color image is reproduced from a continuous-tone original such as a color print, a 35 mm slide, or other film transparency. The majority of output devices, such as printers and plotters, however, cannot reproduce a continuous range of color. With rare exception, computer printers and printing presses simply reproduce either total color tone or nothing. Consequently, the way that the majority of printers or other output devices print variable, continuous ranges of color tone is through the use of half-tone dots to create the impression of various colors. For example, a gray tone can be created by putting a black ink in a pattern of tiny dots and letting the white of the paper show through and mix with the black dots to create an impression of gray. Rendering an image into these dots is called half-toning.
The continuous range of colors seen in most magazines and other color publications is reproduced using half-toning. Conventionally, there are four standard printing ink colors, often referred to as xe2x80x9cprocess colors,xe2x80x9d that have been used in the half-toning process. The four process colors are cyan, magenta, yellow, and black, and are often referred to as xe2x80x9cCMYK,xe2x80x9d where the K stands for black. Together with the background of white paper, the half-tone dots created using these process colors can be used to produce an infinite variety of shades and color tones. Other types of process color schemes have been developed including RGB (red, green, blue), CMY, CMYKOG, and CMYK and any number of spots.
Color image reproductions are typically prepared using separate color prints or plates for each of the process colors. Thus, in order to print a color image it must typically be separated into four half-tone images, one for each of the standard process colors. Electronic separation systems are available which analyze the color content of a digitalized image and separate the image into its constituent process colors. The result of the separation process is four separate prints or films. Once the color separation prints or films have been prepared, they are then utilized to prepare composite color image reproductions.
The degree of sharpness or resolution of the color reproduction prepared by half-toning is a function of a variety of factors. One factor is the screen density or number of dots per linear inch which is used. The human eye can effectively blend half-tone dots with a high degree of resolution at screen densities 130-150 dots per linear inch (dpi). Lower resolution color image productions are also commonly utilized based on screen densities of 85-100 dpi, as for example in many color photographs and charts reproduced in the newspapers.
The quality of a color image reproduction is determined by the degree of resolution or the sharpness of the reproduced images as well as how faithfully the reproduced image corresponds to color in the original image. This in turn means that the quality of the separation is of primary concern. Not only must the color of the various color separation prints or films be faithfully reproduced, but the alignment of the various separation prints is a critical factor. Furthermore, the manner in which the half-tone dots of each of the color separation prints are prepared can also dramatically affect how faithfully the original colors are reproduced as well as the clarity or sharpness of the image. For example, the color density of a particular color is controlled in existing half-toning processes by increasing the size of the dots. The half-tone dots are maintained in a fixed pattern so that they are equally spaced, however, the size of the half-tone dots will be varied in order to selectively increase or decrease the density of a particular color.
A common problem with existing half-toning processes is the over-saturation of the media by the ink. This may result in the ink soaking through the paper, or, worse, running along the paper surface. Another problem that has arisen with existing half tone processes is that there are typically only two levels or types of outputs for each dot in each of the color image. The dot was either on or off. Attempts to resolve some of the above problems included incorporating the use of a modified error diffusion calculation to determine when and where to print a fixed sized dot. This method, however, was only able to determine if the printed dot should be on or off. Thus, even this method was unable to handle multiple levels and stages. As a result, the clarity of the image suffered, dark spots appeared on the reproductions, and the ink saturated the media or, worse, even ran. Further, because the user did not have the ability to control the amount of ink output, the high resolution images required significant processing time as well as printing time.
What is needed in the art is a system and method of color image reproduction which has the ability to convert continuous tone data into multi-tone data and to handle multiple levels of output. There is also a need for a system and method of color image reproduction which eliminates the dark spots and shadows in the image, reduces over-saturation and ink problems. A system and method of color image reproduction is also needed that can be used to reduce the required printing time but still produce high resolution reproductions.
It is an object of the present invention to provide a system and method for color image reproduction utilizing a digital computing system capable of producing multi-level color values as output data.
It is another object of the present invention to provide a system and method for color image reproduction which is capable of generating multi-level color values which can be assigned to digital half tone values.
Another object of the present invention is to generate multi-level color values as output data for each pixel of an image.
Yet another object of the present invention is to provide a method capable of allowing the user to limit the amount of ink that is output by an output device thereby eliminating over-saturation and ink loss.
A further object of the present invention is to provide a method that is capable of handling multiple levels of output.
A further object of the present invention is to provide a method in which the user may specify the number of levels and sublevels depending on the capabilities of a particular printer.
Another object of the present invention is to provide a method of image reproduction that can be used to reduce the time to print an image.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method for color image reproduction is provided that includes inputting an image that is defined by a plurality of pixels. A first array of digital values representing the density and placement of the continuous tone colors contained in each pixel of said image is generated. The first array of digital values is processed so as to derive a separate array of digital multi-level color values representing the density and placement of each color for each of the pixels in the image. The processing generates levels and percentage of ink placements for each level change. In one embodiment, the level and percentage of placement within a level change for each of the pixels is derived using an error diffusion method. The error diffusion method includes digitally processing a separate entry array for each multi-level color value so that for each pixel a separate error diffusion array is generated for a plurality of surrounding pixels. The calculations continue on each error diffusion array so as to assign a weight value to each of the plurality of surrounding pixels. The weight values are dependent upon the relative position of said surrounding pixels of which the error diffusion array was determined. The error diffusion processing use the weight values and the error diffusion array to calculate an output array for each pixel that defines the level and the background of a corresponding pixel. In an alternate embodiment, the separate arrays of digital values representing the density and placement of each multi-level color value for each of the pixels in the image are processed using a half-tone threshold arrays to create an output array for each pixel which defines the levels and background level.
In addition, in one embodiment, the inventive method is capable of limiting the number of sublevels that will be printed thereby allowing the user to control the output, including the amount of ink that is placed on the media. An output array is then produced for each of the colors in the color image.